The author, who suffers from early-onset Alzheimer’s disease, accused opponents – especially Churches – of relying on arguments about “gas chambers” and “jackboots”.

He told a gathering of supporters of assisted dying in Zurich, Switzerland – home of the Dignitas clinic – that the steady stream of people from the UK travelling abroad to end their lives was the “shame of Britain”.

Sir Terry, the creator or the discworld fantasy books, attracted a furore in some quarters last year with a BBC documentary in which he followed Peter Smedley, a 71-year-old motor-neurone sufferer to the clinic to end his life.

He told the congress of the World Federation of Right to Die Societies that the “vast majority” of people in the UK supported assisted suicide but that politicians were cowed by a vocal minority of opponents.

“I have spoken about this many times and always there is vociferous opposition to the idea and ultimately the opposition is from a small number of people, usually associated with the churches,” said Sir Terry.

“Politicians take no notice nevertheless, and the opponents fill the air with dire warnings that elderly people would be persuaded to opt for an early death so as to release their money to the younger generation.,” he said.

“The opposition is very good at asking questions, and absolutely very bad at listening to answers, a usual reaction to any suggestion being it would lead to the gas chambers, and amazingly some newspapers in England print this stuff without questioning how likely it is that the most stable of all democracies on the planet would allow the wholesale killing of the innocent.”

He said that the BBC had received 750 letters of complaints about the programme before it had even been shown.

“Remarkably the wording of the letters had a consistent familiarity, this is not dialogue,” he said.

“You cannot have dialogue with somebody who thinks that shouting the word “Jackboots” is an argument.”